Project Summary/ Abstract Despite evidence that physical activities (PA) targeting leg strength and balance reduce falls, less than 12% of older adults engage in these activities on a regular basis and fall rates continue to increase. Research examining intervention strategies that motivate older adults to engage in PA is scarce and inconclusive. The proposed research aims to address this gap in fall prevention science. The objective of this proposal is to identify behavioral change strategies that elicit increased PA, sustained for 12 months and, in turn, lead to a reduction in falls and improved quality of life (QOL). Building on prior research, the proposed study will use a factorial experimental design to test the relative influence of behavioral change strategies separated into two components. These components are comprised of distinct sets of behavior change strategies that represent two different ways of motivating people to take action: inter-personal and intra-personal, consistent with theories of behavioral change and life-span development, as well as empirical evidence. These components will be combined with core intervention content that includes Otago, an evidence-based, fall-reducing PA protocol, and a PA-monitor for self-tracking. We will recruit 308 community-dwelling adults > 70 years old and randomly assign them to one of four conditions: (a) Otago + PA-monitor; (b) Otago + PA-monitor + inter- personal component; (c) Otago + PA-monitor + intra-personal component; and (d) Otago + PA-monitor + inter-personal + intra-personal components. Content within all conditions will be delivered over 8 weeks to small groups in community centers. We will re-assess all participants post-intervention: immediately, 6 months and 12 months to evaluate the short and long-term efficacy of the intervention components. We hypothesize that participants receiving the inter-personal component, relative to participants not receiving this component, will have a) clinically meaningful increases in PA at all 3 follow-up time points; and b) reductions in falls and improved quality of life 12 months post-intervention. We will also explore the effects of intervention components on targeted mechanisms, motivational constructs (social support; readiness; self- regulation) and physical markers of fall risk (functional balance and strength), and whether these mechanisms are associated with PA and fall outcomes. We hypothesize that receiving the inter-personal motivational component, relative to not receiving this component, will elicit increases in motivational constructs and physical markers of fall risk, which will mediate the intervention?s effects on PA and falls. The proposed research is innovative, because it represents a substantive expansion of biomechanically-oriented fall-reducing PA protocols to a) examine which behavioral change strategies elicit sustained effects (e.g., 12 months) on increased PA, as well as on falls, and QOL; and b) explore underlying mechanisms of PA and falls. The long-term intent of the proposed research is to optimize a PA intervention for older adults that can be used by community-based organizations to promote PA, reduce falls, and improve QOL.